The present invention relates to an apparatus and a method for detecting the state of polarization (SOP) of light.
There are various ways of describing polarization of light, among which the Stokes parameter method is representative. The Stokes parameters are comprised of four elements, i.e., S0, S1, S2 and S3. S0 is the total intensity of light, S1 is the intensity of 0-degree linear polarization minus the intensity of 90-degree linear polarization, S2 is the intensity of 45-degree linear polarization minus the intensity of xe2x88x9245-degree linear polarization, S3 is the intensity of right-hand circular polarization minus the intensity of left-hand circular polarization. When the incident light E(t) is expressed as in Equation 1 below, the Stokes parameters can be obtained by Equation 2 below:
E(t)={circumflex over (x)}Ex(t)eixcfx86x(t)+ŷEy(t)exe2x88x92ixcfx86y(t)xe2x80x83xe2x80x83Equation 1,
where both Ex and Ey are real numbers; and
S0= less than Ex2 greater than + less than Ey2 greater than 
S1= less than Ex2 greater than xe2x88x92 less than Ey2 greater than 
S3= less than 2ExEy cos xcex7 greater than 
S4= less than 2ExEy sin xcex7 greater than xe2x80x83xe2x80x83Equation 2,
where xcex7=xcfx86xxe2x88x92xcfx86y and  less than  greater than  is time average.
The method of measuring the Stokes parameters can. be divided into two types, i.e., the space division measurement method and the time division measurement method.
FIG. 1 is a diagram illustrating the space division measurement method.
Referring to FIG. 1, a first photo detector 30a measures the total intensity lo of the light coming out of a 4-way beam splitter 10 without a polarizer. In this manner, a linear polarizer 20a is aligned at a 0-degree direction in front of a second photo detector 30b, and a linear polarizer 20b is aligned at a 45-degree direction in front of a third photo detector 30c. In this manner, the second and third photo detectors 30b and 30c measure the intensities of the respective linear polarization components I1 and I2 of each direction. A fourth photo detector 30d measures the intensity I3 of the light that has propagated through the 1/4 wavelength phase retarder 15 aligned at a 45-degree direction and a linear polarizer 20c aligned at a 0-degree direction. The intensity I3 corresponds to the intensity of the right-hand circular polarization. The measured intensities I0, I1, I2,and I3 have a relationship with the Stokes parameter as follows:
S0=I0
S1=2I1xe2x88x92I0
S2=2I2xe2x88x92I0
S2=2I2xe2x88x92I0xe2x80x83xe2x80x83Equation3.
According to the method illustrated in FIG. 1, the measurement can be done rapidly because light is equally divided into four components and measured at the same time by use of four photo detectors. However, the apparatus is complicated and the sensitivity can vary with photo detectors, because four photo detectors are used.
FIG. 2 is a diagram illustrating the time division measurement method. Referring to FIG. 2, a phase retarder 200 and a polarizer 210 which are capable of rotation to a certain angle are located sequentially in front of-a photo detector 220. By adjusting the angle, it is possible to measure the light intensities I0, I1, I2, and I3 that have a relation with the Stokes parameters as illustrated in Equation 3. However, this method has a disadvantage that the relative angle between the phase retarder 200 and the polarizer 210 must be very precisely controlled.
The above-illustrated space division or time division measurement methods are capable of modification. However, the disadvantage of these methods is that they use a 1/4 wavelength retarder as the phase retarder in which error easily occurs, because the amount of retardation is a function of wavelength.
Therefore, it is an object of the present invention is to provide an apparatus for detecting polarization with a simplified structure.
It is another object of the present invention to provide a method for detecting polarization with an enhanced speed.
It is still another object of the present invention to provide a method of detecting the state of polarization of light in a wide range of wavelength by reducing errors in detecting the state of polarization. In particular, it is the object of the present invention to reduce the measurement error arising from the change in the amount of phase retardation with respect to the wavelength in the phase retardation plate.
To this end, an apparatus for detecting polarization is provided, the apparatus comprising a phase retarder rotating at a first constant speed for causing phase retardation between polarization components of an incident light with respect to a fast axis and a slow axis of the phase retarder, a polarizer rotating at a second constant speed for changing a state of polarization of light that propagated through the phase retarder, a detector for detecting an intensity of light that propagated through the polarizer, and a signal processing unit for deriving frequency components of electrical signals of light that was detected at the detector to analyze the state of polarization of the incident light.
Preferably, the phase retarder is a 1/4 wavelength plate.
The method of detecting a state of polarization of an incident light according to the present invention comprises the steps of periodically changing the state of polarization of polarization components of the incident light by propagating the incident light through a phase retarder that has a fast axis and a slow axis and rotates at a first constant speed, changing an intensity of the incident light by propagating the incident light of which the state of polarization was changed through a polarizer rotating at a second constant speed, detecting the changed intensity of the incident light, and deriving frequency components from the detected intensity of the incident light to obtain Stokes parameters.
Preferably, the first constant speed and the second constant speed are different from each other. Alternatively, the first constant speed and the second constant speed may have same magnitudes but opposite directions.
Still alternatively, the first constant speed and the second constant speed may have same directions and the ratio of a magnitude of the first constant speed to a magnitude of the second constant speed may range from 1/3 to 3/1.